The broad objectives of the proposed project are to investigate the functional and comparative morphology and the ultrastructural details of neurons, axons, and synapses in simple phylogenetically primitive and evolutionarily more complex nervous systems. The combined electron microscopical approaches of scanning, conventional transmission and high voltage electron microscopy will be used to analyze and reconstruct in three dimensions simple neuronal pathways and connectivity patterns between neurons and other cells. The objectives of this research are threefold: (1) to elucidate the developement and evolution of the nervous system, (2) to provide comparative ultrastructural information on various types of synaptic contacts in simple and more complex nervous systems. and (3) to evaluate the potential advantages of combining serial section reconstruction and electron stereomicroscopy to a study of the nervous system. It is expected that the results from the technical approaches used for this study will provide new insight into alternative electron microscopic methodology for the study of normal and pathological states of the central nervous system and associated muscular diseases in humans, as well as for biomedical research in general. The results of an ultrastructural comparison of synaptic contacts in invertebrate and vertebrate systems may assist in clarification of a logical hypothesis of evolution of neurons and synapses in the animal kingdom.